Spread spectrum adaptive power control

ABSTRACT

A system and method for adaptive power control of a spread spectrum transmitter of a mobile unit operating in a cellular-communications network having a plurality of mobile units in communication with a base station. In response to a received signal from the mobile unit, the received signal having a first spread-spectrum signal and an interfering signal, an automatic gain control (AGC) circuit within the base station generates an AGC-output signal which is despread by a base despreader and then processed as a received-power level. A power command signal is generated using a threshold test and the received-power level. The power command signal is transmitted to the mobile station as a second spread-spectrum signal. The mobile station despreader despreads the second spread-spectrum signal as a power adjust signal and, responsive to the power adjust signal, increases or decreases the power level of the first spread spectrum signal.

[0001] This application is a continuation of U.S. patent applicationSer. No.10/027,693, filed Oct. 19, 2001, which is a continuation of U.S.patent application Ser. No. 09/716,864, filed Nov. 20,2000, which issuedon Dec. 11,2001 as U.S. Pat. No.6,330,272, which is a continuation ofU.S. patent application Ser. No. 09/277,400, filed Mar. 26, 1999, whichissued on Jan. 16, 2001 as U.S. Pat. No. 6,175,586, which is acontinuation of U.S. patent application Ser. No.08/891,236, filed Jul.10, 1997, which issued on Nov. 30, 1999 as U.S. Pat. No. 5,995,538,which is a divisional of U.S. patent application Ser. No. 08/743,379,filed Nov. 4,1996, which issued on Nov. 10, 1998 as U.S. Pat. No.5,835,527, which is a continuation of U.S. patent application Ser.No.08/368,710, filed Jan. 4, 1995, which issued on Nov. 12, 1996 as U.S.Pat. No. 5,574,747.

BACKGROUND

[0002] This invention relates to spread-spectrum communications, andmore particularly to a multipath processor, variable bandwidth device,and power control system.

[0003] Spread-spectrum modulation provides means for communicating inwhich a spread-spectrum signal occupies a bandwidth in excess of theminimum bandwidth necessary to send the same information. The bandspread is accomplished by modulating an information-data signal with achipping-sequence signal which is independent of an information-datasignal. The information-data signal may come from a data device such asa computer, or an analog device which outputs an analog signal which hasbeen digitized to an information-data signal, such as voice or video.The chipping-sequence signal is generated by a chip-code where the timeduration, T_(c), of each chip is substantially less than a data bit ordata symbol. A synchronized reception of the information-data signalwith the chipping-sequence signal at a receiver is used for despreadingthe spread-spectrum signal and subsequent recovery of data from thespread-spectrum signal.

[0004] Spread-spectrum modulation offers many advantages as acommunications system for an office or urban environment. Theseadvantages include reducing intentional and unintentional interference,combating multipath problems, and providing multiple access to acommunications system shared by multiple users. Commercially, theseapplications include, but are not limited to, local area networks forcomputers and personal communications networks for telephone, as well asother data applications.

[0005] A cellular communications network, using spread-spectrummodulation for communicating between a base station and a multiplicityof users, requires control of the power level of a particular mobileuser station. Within a particular cell, a mobile station near the basestation of the cell may be required to transmit with a power level lessthan that required when the mobile station is near an outer perimeter ofthe cell. This adjustment in power level is done to ensure a constantpower level is received at the base station from each mobile station.

[0006] In a first geographical region, such as an urban environment, thecellular architecture may have small cells in which the respective basestations are close to each other, requiring a low power level from eachmobile user. In a second geographical region, such as a ruralenvironment, the cellular architecture may have large cells in which therespective base stations are spread apart, requiring a relatively highpower level from each mobile user. A mobile user who moves from thefirst geographical region to the second geographical region typicallyadjusts the power level of his transmitter in order to meet therequirements of a particular geographic region. If such adjustments werenot made, a mobile user traveling from a sparsely populated region withlarger cells, using the relatively higher power level with hisspread-spectrum transmitter, to a densely populated region with manysmall cells may, without reducing the original power level of hisspread-spectrum transmitter, cause undesirable interference within thesmaller cell into which he has traveled and/or to adjacent cells. Also,if a mobile user moves behind a building and has his signal to the basestation blocked by the building, then the mobile user's power levelshould be increased. These adjustments must be made quickly, with highdynamic range and in a manner to ensure an almost constant receivedpower level with low root mean square error and peak deviations from theconstant level.

[0007] Accordingly, there is a need to have a spread-spectrum system andmethod for automatically controlling a mobile user's spread-spectrumtransmitter power level when operating in a cellular communicationsnetwork.

SUMMARY

[0008] A system and method for adaptive power control of a spreadspectrum transmitter of a mobile unit operating in acellular-communications network having a plurality of mobile units incommunication with a base station. In response to a received signal fromthe mobile unit, the received signal having a first spread-spectrumsignal and an interfering signal, an automatic gain control (AGC)circuit within the base station generates an AGC-output signal which isdespread by a base despreader and then processed as a received-powerlevel. A power command signal is generated using a threshold test andthe received-power level. The power command signal is transmitted to themobile station as a second spread-spectrum signal. The mobile stationdespreader despreads the second spread-spectrum signal as a power adjustsignal and, responsive to the power adjust signal, increases ordecreases the power level of the first spread spectrum signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0009] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0010]FIG. 1 illustrates channel impulse response giving rise to severalmultipath signals;

[0011]FIG. 2 illustrates conditions leading to two groups of severalmultipath signals;

[0012]FIG. 3 is a block diagram of a multipath processor using two setsof correlators for despreading a spread-spectrum signal received as twogroups of spread-spectrum signals;

[0013]FIG. 4 is a block diagram for generating chipping-sequence signalswith delays;

[0014]FIG. 5 is a tapped-delay line model of a communications channel;

[0015]FIG. 6 is a block diagram of a correlator;

[0016]FIG. 7 is an auto correlation function diagram generated from thecorrelator of FIG. 6;

[0017]FIG. 8 is a block diagram for tracking a received signal;

[0018]FIG. 9 is a block diagram for combining a pilot signal from areceived spread-spectrum signal;

[0019]FIG. 10 is a block diagram for tracking a pilot signal embedded ina pilot channel of a spread-spectrum signal;

[0020]FIG. 11 illustrates cross-correlation between a received signaland a referenced chipping-sequence signal, as a function of referenceddelay;

[0021]FIG. 12 illustrates the center of gravity of the cross-correlationfunction of FIG. 11;

[0022]FIG. 13 is a block diagram of a multipath processor using two setsof matched filters for despreading a spread-spectrum signal received astwo groups of spread-spectrum signals;

[0023]FIG. 14 is a block diagram of a multipath processor using threesets of correlators for despreading a spread-spectrum signal received asthree groups of spread-spectrum signals;

[0024]FIG. 15 is a block diagram of a multipath processor using threesets of matched filters for despreading a spread-spectrum signalreceived as three groups of spread-spectrum signals;

[0025]FIG. 16 is a block diagram of a variable-bandwidth spread-spectrumdevice;

[0026]FIG. 17 illustrates chips of a spread-data signal;

[0027]FIG. 18 illustrates impulse signals corresponding to the chips ofthe spread-data signal of FIG. 17;

[0028]FIG. 19 is an alternative block diagram of the variable-bandwidthspread-spectrum device of FIG. 16;

[0029]FIG. 20 is a block diagram of a base station;

[0030]FIG. 21 is a block diagram of a mobile station;

[0031]FIG. 22 illustrates nonlinear power adjustment;

[0032]FIG. 23 illustrates linear and nonlinear cower adjustment;

[0033]FIG. 24 illustrates fades during transmission for multiple signalsor equivalent power received at a base station;

[0034]FIG. 25 illustrates an adaptive power control signal of broadcastpower for a fixed step algorithm;

[0035]FIG. 26 illustrates despread output power for a fixed stepalgorithm;

[0036]FIG. 27 illustrates an adaptive power control signal of broadcastpower for a variable step algorithm; and

[0037]FIG. 28 illustrates despread output power for a variable stepalgorithm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0038] Reference now is made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

Multipath Processor

[0039] In a multipath environment, a signal reflects from severalbuildings or other structures. The multiple reflections from the severalbuildings can result in several signals, or several groups of signals,arriving at a receiver. FIG. 1 illustrates a signal arriving in time asseveral signals. FIG. 2 illustrates a signal arriving in time as twogroups of several signals. The multiple signals arriving at the receiverusually do not arrive with a uniform spread over time. Thus, in amultipath environment, a received signal r(t) may include two or moregroups of spread-spectrum signals.

[0040] In the multipath environment, a spread-spectrum signal is assumedto generate a plurality of groups of spread-spectrum signals, with eachgroup having a plurality of spread-spectrum signals. The plurality ofgroups is the result of the spread-spectrum signal reflecting in amultipath environment. As a means of responding to and dealing with thisplurality of groups, the multipath processor is an improvement to aspread-spectrum receiver system. In the exemplary arrangement shown inFIG. 3, a multipath processor for tracking a spread-spectrum signal isshown. The multipath processor is used as part of a spread-spectrumreceiver system.

[0041] The multipath processor includes first despreading means, seconddespreading means, first combining means, second combining means, andselecting means or output-combining means. The first combining means iscoupled between the first despreading means and the selecting means orthe output-combining signal. The second combining means is coupledbetween the second despreading means and the selecting means or theoutput-combining means.

[0042] The first despreading means despreads a received signal having afirst plurality of spread-spectrum signals within a first group. Thefirst despreading means thus generates a first plurality of despreadsignals. The first combining means combines, or adds together, the firstplurality of despread signals to generate a first combined-despreadsignal.

[0043] The second despreading means despreads the received signal havinga second plurality of spread-spectrum signals within a second group. Thesecond despreading means thereby generates a second plurality ofdespread signals. The second combining means combines, or adds together,the second plurality of despread signals as a second combined-despreadsignal.

[0044] The selecting means selects either the first combined-despreadsignal or the second combined-despread signal. The selectedcombined-despread signal is outputted from the selecting means as anoutput-despread signal. The selecting means may operate responsive tothe stronger signal strength of the first combined-despread signal andthe second combined-despread signal, least mean square error, a maximumlikelihood, or other selection criteria. Alternatively, usingoutput-combining means in place of selecting means, the outputs of thefirst combining means and the second combining means may be coherentlycombined or added together, after suitable weighting.

[0045] As shown in FIG. 3, the first despreading means may include afirst plurality of correlators for despreading, respectively, the firstplurality of spread-spectrum signals. The first plurality of correlatorsis illustrated, by way of example, as first multiplier 111, secondmultiplier 112, third multiplier 113, first filter 121, second filter122, third filter 123, first chipping-sequence signal g(t), secondchipping-sequence signal g(t-T_(o)), and third chipping-sequence signalg(t-2T_(o)). The second chipping-sequence signal g(t-T_(o)) and thethird chipping-sequence signal g(t-2T_(o)) are the same as the firstchipping-sequence signal g(t), but delayed by time T_(o) and time2T_(o), respectively. The delay between each chipping-sequence signal,preferably, is a fixed delay T_(o). At the input is received signalr(t). The first multiplier 111 is coupled between the input and thefirst filter 121, and to a source of the first chipping-sequence signalg(t). The second multiplier 112 is coupled between the input and thesecond filter 122, and to a source of the second chipping-sequencesignal g(t-T_(o)). The third multiplier 113 is coupled between the inputand the third filter 123, and to a source of the third chipping-sequencesignal g(t-2T_(o)). The outputs of the first filter 121, the secondfilter 122 and the third filter 123 are coupled to the first adder 120.

[0046] Circuitry and apparatus are well known in the art for generatingchipping-sequence signals with various delays. Referring to FIG. 4, achipping-sequence generator 401 is coupled to a voltage-controlledoscillator 402, and a plurality of delay devices 403, 404, 405, 406. Thevoltage-controlled oscillator receives a group-delay signal. Thegroup-delay signal corresponds to the time delay that the group ofchipping-sequence signals used for despreading a particular group ofreceived signals. The voltage-controlled oscillator 402 generates anoscillator signal. The chipping-sequence generator 401 generates thefirst chipping-sequence signal g(t) from the oscillator signal, with aninitial position of the first chipping-sequence signal g(t) determinedfrom the group-delay signal. The first chipping-sequence signal g(t) isdelayed by the plurality of delay devices 403, 404, 405, 406, togenerate the second chipping-sequence signal g(t-T_(o)), the thirdchipping-sequence signal g(t-2T_(o)), the fourth chipping-sequencesignal (gt-3T_(o)), etc. Thus, the second chipping-sequence signalg(t-2T_(o)) and the third chipping-sequence signal g(t-2T_(o)) may begenerated as delayed versions of the first chipping-sequence signalg(t). Additionally, acquisition and tracking circuitry are part of thereceiver circuit for acquiring a particular chipping-sequence signalembedded in a received spread-spectrum signal.

[0047] Optionally, the multipath processor of FIG. 3 may include firstweighting device 131, second weighting device 132 and third weightingdevice 133. The first weighting device 131 is coupled to the output ofthe first filter 121, and a source of a first weighting signal W₁. Thesecond weighting device 132 is coupled to the output of the secondfilter 122, and to a source of the second weighting signal W₂. The thirdweighting device 133 is coupled to the output of the third filter 123and to a source of the third weighting signal W₃. The first weightingsignal W₁, the second weighting signal W₂ and the third weighting signalW₃ are optional, and may be preset within the first weighting device131, the second weighting device 132 and the third weighting device 133,respectively. Alternatively, the first weighting signal W₁, the secondweighting signal W₂, and the third weighting signal W₃ may be controlledby a processor or other control circuitry. The outputs of the firstfilter 121, the second filter 122, and the third filter 123 are coupledthrough the first weighting device 131, the second weighting device 132and the third weighting device 133, respectively, to the first adder120.

[0048] Similarly, the second despreading means may include a secondplurality of correlators for despreading the second plurality ofspread-spectrum signals. The second plurality of correlators isillustrated, by way of example, as fourth multiplier 114, fifthmultiplier 115, sixth multiplier 116, fourth filter 124, fifth filter125, sixth filter 126, fourth chipping-sequence signal g(t-T_(D1)),fifth chipping-sequence signal g(t-T_(o)-T_(D1)), and sixthchipping-sequence signal g(t-2T_(o)-T_(D1)). The fourth multiplier 114is coupled between the input and the fourth filter 124, and a source ofthe fourth chipping-sequence signal g(t-T_(D1)). The fifth multiplier115 is coupled between the input and the fifth filter 125 and a sourceof the fifth chipping-sequence signal g(t-T_(o)-T_(D1)). The sixthmultiplier 116 is coupled between the input and the sixth filter 126,and a source of the sixth chipping-sequence signal g(t-2T_(o-T) _(D1)).The fourth chipping-sequence signal g(t-T_(D1)), the fifthchipping-sequence signal g(t-T_(o-T) _(D1)) and the sixthchipping-sequence signal g(t-2T_(o)-T_(D1)) are the same as the firstchipping-sequence signal g(t), but delayed by time T_(D1) timeT_(o)+T_(D1), and time 2T_(o)+T_(D1), respectively. The second pluralityof correlators thereby generates the second plurality of despreadsignals. The outputs of the fourth filter 124, the fifth filter 125 andthe sixth filter 126 are coupled to the second adder 130.

[0049] At the output of the fourth filter 124, the fifth filter 125, andthe sixth filter 126, optionally, may be fourth weighting device 134,fifth weighting device 135, and sixth weighting device 136. The fourthweighting device 134, fifth weighting device 135, and sixth weightingdevice 136 are coupled to a source which generates fourth weightingsignal W₄, fifth weighting signal W₅, and sixth weighting signal W₆,respectively. The fourth weighting signal W₄, the fifth weighting signalW₅, and the sixth weighting signal W₆ are optional, and may be presetwithin the fourth weighting device 134, the fifth weighting device 135,and the sixth weighting device 136, respectively. Alternatively, thefourth weighting signal W₄, the fifth weighting signal W₅, and the sixthweighting signal W₆ may be controlled by a processor or other controlcircuitry. The outputs of the fourth filter 124, fifth filter 125, andsixth filter 126 are coupled through the fourth weighting device 134,fifth weighting device, 135 and sixth weighting device 136,respectively, to the second adder 130. The output of the first adder 120and the second adder 130 are coupled to the decision device 150. Thedecision device 150 may be a selector or a combiner.

[0050] The weighting devices maybe embodied as an amplifier orattenuation circuits, which change the magnitude and phase. Theamplifier or attenuation circuits may be implemented with analog devicesor with digital circuitry. The amplifier circuit or attenuation circuitmay be adjustable, with the gain of the amplifier circuit or attenuationcircuit controlled by the weighting signal. The use of a weightingsignal with a particular weighting device is optional. A particularweighting device may be designed with a fixed weight or a preset amount,such as a fixed amount of amplifier gain.

[0051]FIG. 5 is a tapped-delay-line model of a communications channel. Asignal s(t) entering the communications channel passes through aplurality of delays 411,412,413,414, modeled with time T_(o). The signals(t), for each delay, is attenuated 416, 417, 418 by a plurality ofcomplex attenuation factors h_(n−1), h_(n), h_(n+) and adder 419. TheOUTPUT from the adder 419 is the output from the communications channel.

[0052] A given communications channel has a frequency response which isthe Fourier transform of the impulse response.${H(f)} = {\sum\limits_{i = 1}^{N}\quad {a_{i}e^{{- {j2}}\quad \pi \quad {f\tau}_{i}}}}$

[0053] where a_(i) represents the complex gains of the multipaths of thecommunications channel, and t_(i) represents the delays of themultipaths of the communications channel.

[0054] Consider the communications-channel-frequency response, H_(c)(f).The communications-channel-frequency response has a band of interest, B.Hereafter, this band of interest is fixed, and thecommunications-channel-frequency response H_(c)(f) is the equivalentlowpass filter function. The communications-channel-frequency responseexpands in Fourier series as

H _(c)(f)=Σh _(n) e ^(−jn2πf/B)

[0055] where h_(n) represents Fourier coefficients. This is atapped-delay-line model of the communications channel for which thereceiver in FIG. 3 acts as a matched filter when T_(o)=1/B, and theweights W_(n) are set to the complex conjugate of the values h_(n). Thatis, W_(n)=h_(n).

[0056] Preferably, each correlator of the first plurality of correlatorsdespreads with a chipping-sequence signal g(t) which has a time delaydifferent from each time delay of each chipping-sequence signal used,respectively, with each of the other correlators of the first pluralityof correlators. The first plurality of correlators useschipping-sequence signals g(t), g(t-T_(o)), g(t-2T_(o)), where T_(o) isthe time delay between chipping-sequence signals. The time delay T_(o)may be the same or different between each chipping-sequence signal. Forillustrative purposes, time delay T_(o) is assumed to be the same.

[0057] Similarly, each correlator of the second plurality of correlatorsdespreads with a chipping-sequence signal having a time delay differentfrom each time delay of each other chipping-sequence signal used,respectively, with each of the other correlators of the second pluralityof correlators. Also, each correlator of the second Plurality ofcorrelators despreads with a chipping-sequence signal having the timedelay T_(D1) different from each time delay of each chipping-sequencesignal used with each respective correlator of the first plurality ofcorrelators. Thus, the second plurality of correlators useschipping-sequence signals g(t-T_(D1)), g(t-T_(o)-T_(D1)),g(t-2T_(o)-T_(D1)), where time delay T_(D1) is the time delay betweenthe first plurality of correlators and the second plurality ofcorrelators. The time delay T_(D1) is also approximately the same timedelay as between the first received group of spread-spectrum signals andthe second received group of spread-spectrum signals.

[0058]FIG. 6 illustrates a correlator, where an input signal s(t) ismultiplied by multiplier 674 by a delayed version of the input signals(t-T). The product of the two signals is filtered by the filter 675,and the output is the autocorrelation function R(T). The autocorrelationfunction R(T) for a square wave input signal s(t) is shown in FIG. 7.Over a chip time T_(c), the correlation function R(T) is maximized whenpoints A and B are equal in amplitude. A circuit which is well known inthe art for performing this function is shown in FIG. 8. In FIG. 8, thedespread signal s(t) is delayed by a half chip time T_(c/2), andforwarded by half a chip time T_(c/2). Each of the three signals aremultiplied by the received signal r(t). The outputs of the delayed andforwarded multiplied signals are filtered, and then amplitude detected.The two filtered signals are combined by subtracting the delayed versionfrom the forwarded version, and the difference or error signal is usedto adjust the timing of the chipping-sequence signal used to despreadsignal s(t). Accordingly, if the delayed version were ahead of theforwarded version, the chipping-sequence signal for despread signal s(t)would be delayed. Likewise, if the forwarded version were ahead of thedelayed version, then the chipping-sequence signal for despreadingsignal s(t) would be advanced. These techniques are well known in theart.

[0059] A similar technique is used for estimating a pilot signal from areceived signal r(t), which has passed through a multipath environment.Referring to FIG. 9, the lower part of the diagram shows correlatorscorresponding to the correlators previously shown in FIG. 3. The upperpart of the diagram shows the received signal processed by delayedversions or the pilot chipping-sequence signal g_(p)(t). In FIG. 9, thereceived signal r(t) is multiplied by the pilot signal g_(p)(t) and aplurality of delayed versions of the pilot signal g p (t-T_(o)),g_(p)(t-kT_(o)) by a plurality of multipliers 661, 651, 641. The outputof the plurality of multipliers 601, 651, 641, are each filtered by aplurality of filters 662, 652, 642, respectively. The output of theplurality of filters 662, 652, 642 are multiplied by a second pluralityof multipliers 663, 653, 643 and respectively filtered by a secondplurality of filters 664, 654, 644. The outputs of the second pluralityof filters 664, 654, 644 are processed through a plurality of complexconjugate devices 665, 655, 645. The outputs of the plurality of complexconjugate devices 665, 655, 645 are the plurality of weights W₁, W₂,W_(k), respectively. The plurality of weights are multiplied by theoutput of the first plurality of filters 662, 652, 642, by a thirdplurality of multipliers 666, 656, 646, and then combined by thecombiner 667. At the output of the combiner 667 is acombined-despread-pilot signal.

[0060] Each of the second plurality of pilot filters 664, 654, 644 has abandwidth which is approximately equal to the fading bandwidth. Thisbandwidth typically is very narrow, and may be on the order of severalhundred Hertz.

[0061] Referring to FIG. 10, the output of the combiner 667 ismultiplied by a fourth multiplier 668, and passed through an imaginarydevice 669 for determining the imaginary component of the complex signalfrom the fourth multiplier 668. The output of the imaginary device 669passes through a loop filter 672 to a voltage controlled oscillator 673or a numerically controlled oscillator (NCO). The output of the voltagecontrolled oscillator 673 passes to the fourth multiplier 668 and toeach or the second plurality of multipliers 663, 653, 643.

[0062] Referring to FIG. 11, the foregoing circuits can generate across-correlation function between the received signal and a referencedpilot-chipping signal as a function of referenced delay, or lag. Asshown in FIG. 11, these points of cross-correlation can have a center ofgravity. The center of gravity is determined when the left mass equalsthe right mass of the correlation function, as shown in FIG. 12. Acircuit similar to that shown in FIG. 8, coupled to the output of thefourth multiplier 668, can be used for aligning a chipping-sequencesignal of the pilot channel.

[0063] As an alternative embodiment, as shown in FIG. 13, the firstdespreading means may include a first plurality of matched filters fordespreading the received signal r(t) having the first plurality ofspread-spectrum signals. At the output of the first plurality of matchedfilters is the first plurality oC despread signals. Each matched filterof the first plurality of matched filters has an impulse response h(t),h(t-T_(o)), h(t-2T_(o)), etc., with a time delay To offset from theother matched filters. Referring to FIG. 13, by way of example, a firstmatched filter 141 is coupled between the input and through the firstweighting device 131 to the first adder 120. A second matched filter 142is coupled between the input and through the second weighting device 132to the first adder 120. A third matched filter 143 is coupled betweenthe input and through the third weighting device 133 to the first adder120. As mentioned previously, the first weighting device 131, the secondweighting device 132, and the third weighting device 133 are optional.The first weighting device 131, the second weighting device 132, and thethird weighting device 133 generally are connected to a source of thefirst weighting signal W₁, the second weighting signal W₂, and the thirdweighting signal W₃, respectively. The first plurality of matchedfilters generates the first plurality of despread signals.

[0064] Similarly, the second despreading means may include a secondplurality of matched filters for despreading the received signal r(t)having the second plurality of spread-spectrum signals. Accordingly, atthe output of the second plurality of matched filters is the secondplurality of despread signals. Each matched filter of the secondplurality of matched filters has an impulse response, h(t-T_(D1)),h(t-T_(o)-T_(D1)), h(t-2T_(o)-T_(D1)), etc., with a time delay To offsetfrom the other matched filters and with a time delay T_(D1) offset fromthe first plurality of matched filters. A fourth matched filter 144 iscoupled between the input and through the fourth weighting device 134 tothe second adder 130. A fifth matched filter 145 is coupled between theinput, and through the fifth weighting device 135 to the second adder130. A sixth matched filter 146 is coupled between the input and throughthe sixth weighting device 136 to the second adder 130. As mentionedpreviously, the fourth weighting device 134, the fifth weighting device135, and the sixth weighting device 136 are optional. The fourthweighting device 134, the fifth weighting device 135, and the sixthweighting device 136, are coupled respectively to a source forgenerating the fourth weighting signal W₄, the fifth weighting signalW₅, and the sixth weighting signal W₆. Also, as with the correlatorembodiment, the first adder 120 and the second adder 130 are coupled tothe decision device 150. The decision device 150 may be embodied as aselector or a combiner.

[0065] The present invention may further include despreadingspread-spectrum signals located within a third group. Accordingly, thepresent invention may include third despreading means and thirdcombining means. The third combining means is coupled between the thirddespreading means and the selecting means.

[0066] As shown in FIG. 14, the third despreading means despreads thereceived signal r(t) received as a third plurality or spread-spectrumsignals within a third group. Accordingly, the third despreading meansgenerates a third plurality of despread signals. The third combiningmeans combines the third plurality of despread signals as a thirdcombined-despread signal. The selecting means selects one of the firstcombined-despread signal, the second combined-despread signal or thethird combined-despread signal. The output of the selecting means is theoutput-despread signal.

[0067] As shown in FIG. 14, the third despreading means may include athird plurality of correlators for despreading the third plurality ofspread-spectrum signals. The third plurality of correlators isillustrated, by way of example, with seventh multiplier 117, eighthmultiplier 118, ninth multiplier 119, seventh filter 127, eighth filter128, ninth filter 129, and a source for generating the seventhchipping-sequence signal g(t-T_(D2)), the eighth chipping-sequencesignal g(t-T_(o)-T_(D2)), and the ninth chipping-sequence signalg(t-T_(o)-T_(D2)). The seventh multiplier 117 is coupled between theinput and the seventh filter 127. The eighth multiplier 118 is coupledbetween the input and the eighth filter 128. The ninth multiplier 119 iscoupled between the input and the ninth filter 129. The seventhmultiplier 117, the eighth multiplier 118, and the ninth multiplier 119,are coupled to the source for generating the seventh chipping-sequencesignal, the eighth chipping-sequence signal and the ninthchipping-sequence signal, respectively. Optionally, at the output of theseventh filter 127, eighth filter 128, and ninth filter 129, may beseventh weighting device 137, eighth weighting device 138, and ninthweighting device 139, respectively. Accordingly, the output of theseventh filter 127 is coupled through the seventh weighting device 137to the third adder 140. The output of the eighth filter 128 is coupledthrough the eighth weighting device 138 to the third adder 140. Theoutput of the ninth multiplier 129 is coupled through the ninthweighting device 139 to the third adder 140. The third adder is coupledto the decision device 150. At the output of the third plurality ofcorrelators is the third plurality of despread signals, respectively.

[0068] Preferably, each correlator of the third plurality of correlatorsdespreads with a chipping-sequence signal g(t-T_(D2)),g(t-T_(o)-T_(D2)), g(t-2T_(o)-T_(D2)) having a time delay T_(o)different from each time delay of each chipping-sequence signal usedwith other correlators of the third plurality of correlators. Also, eachcorrelator of the third plurality of correlators despreads with achipping-sequence signal having a time delay different from each timedelay of each chipping-sequence signal used, respectively, with eachcorrelator of the second plurality of correlators. Also, each correlatorof the third plurality of correlators despreads with a chipping-sequencesignal having a time delay 2T_(D) different from each chipping-sequencesignal used with each correlator of the first plurality of correlators.

[0069] Alternatively, the third despreading means may include, as shownin FIG. 15, a third plurality of matched filters for despreading thethird plurality of spread-spectrum signals. The third plurality ofmatched filters includes seventh matched filter 147, eighth matchedfilter 148, and ninth matched filter 149. The seventh matched filter iscoupled between the input and through the seventh weighting device 137to the third adder 140. The eighth matched filter 148 is coupled betweenthe input and through the eighth weighting device 138 to the third adder140. The ninth matched filter 149 is coupled between the input andthrough the ninth weighting device 139 to the third adder 140. The thirdadder 140 is coupled to the decision device 150. At the output of thethird plurality of matched filters is the third plurality of despreadsignals.

[0070] The present invention may include fourth despreading means andfourth combining means, with the fourth combining means coupled betweenthe fourth despreading means and the selecting means. The fourthdespreading means would despread a fourth plurality of spread-spectrumsignals within a fourth group. The output of the fourth despreadingmeans would be a fourth plurality of despread signals. The fourthcombining means would combine the fourth plurality of despread signalsas a fourth combined-despread signal. The selecting means selects one ofthe first combined-despread signal, the second combined-despread signal,the third combined-despread signal, or the fourth combined-despreadsignal, as the output-despread signal.

[0071] In a similar fashion, the fourth despreading means includes afourth plurality of correlators, or a fourth plurality of matchedfilters, for despreading the fourth plurality of spread-spectrum signalsfor generating the fourth plurality of despread signals. Each correlatorof the fourth plurality of correlators would despread with achipping-sequence signal having a time delay different from each timedelay of each chipping-sequence signal used, respectively, with othercorrelators of the fourth plurality of correlators. Also, thechipping-sequence signal would be different from the chipping-sequencesignals used with each correlator of the third plurality of correlators,each chipping-sequence signal used with each correlator of the secondplurality of correlators, and each chipping-sequence signal used witheach correlator of the first plurality of correlators. Based on thedisclosure herein, a person skilled in the art would readily know how toextend the concept to a fifth group or spread-spectrum signals, or moregenerally, to a plurality of groups of spread-spectrum signals.

[0072] Each of the matched filters may be realized usingsurface-acoustic-wave (SAW) devices, digital matched filters, orembodied in an application specific integrated circuit (ASIC) chip or adigital signal processor (DSP) chip. Techniques for designing matchedfilters using these devices are well known in the art.

[0073] A multipath processor can single out individual paths from agroup of rays. The weight for each weighting device is figured out bysets of correlators, and with a reference code it is possible to trackthe chipping-sequence signal in each ray.

[0074] Alternatively, a method using a multipath processor may be usedfor tracking a spread-spectrum signal within a plurality of groups. Themethod comprises the steps of despreading the received signal r(t)received as the first plurality of spread-spectrum signals within afirst group to generate a first plurality of despread signals. The firstplurality of despread signals are then combined as a firstcombined-despread signal. The method would include despreading thereceived signal r(t) received as a second plurality of spread-spectrumsignals within a second group to generate a second plurality of despreadsignals. The second plurality of despread signals would be combined as asecond combined-despread signal. The method includes selecting eitherthe first combined-despread signal or the second combined-despreadsignal, as an output-despread signal.

[0075] The step of despreading the first plurality of spread-spectrumsignals may include the step of correlating or matched filtering thefirst plurality of spread-spectrum signals, using a first plurality ofcorrelators or a first plurality of matched filters, respectively. Thestep of despreading the second plurality of spread-spectrum signals mayinclude the step of correlating or matched filtering the secondplurality of spread-spectrum signals using a second plurality ofcorrelators or a second plurality of matched filters, respectively.

[0076] The method may further include despreading a third plurality ofspread-spectrum signals within a third group to generate a thirdplurality of despread signals. The third plurality of despread signalswould be combined as a third combined-despread signal. The selectingstep would thereby include selecting one of the first combined-despreadsignal, the second combined-despread signal or the thirdcombined-despread signal, as the output-despread signal. Similarly, thestep of despreading the third plurality of spread-spectrum signals mayinclude the step of correlating or matched filtering the third pluralityof spread-spectrum signals using a third plurality of correlators or athird plurality of matched filters, respectively.

[0077] The step of despreading each of the first plurality ofspread-spectrum signals would include the step of despreading with achipping-sequence signal having a time delay different from each timedelay of each chipping-sequence signal used to despread otherspread-spectrum signals of the first plurality of spread-spectrumsignals. Similarly, the step of despreading each of the second pluralityof spread-spectrum signals would include the step of despreading with achipping-sequence signal having a time delay different from each timedelay of each chipping-sequence signal used to despread otherspread-spectrum signals of the second plurality of spread-spectrumsignals. Also, the step of despreading each of the second plurality ofspread-spectrum signals would include the step of despreading with achipping-sequence signal having a time delay different from each timedelay of each chipping-sequence signal used to despread otherspread-spectrum signals of the first plurality of spread-spectrumsignals.

[0078] In the event the method includes the step of despreading a thirdplurality of spread-spectrum signals, the method would include the stepsof despreading with a chipping-sequence signal having a time delaydifferent for each time delay of each chipping-sequence signal used todespread other spread-spectrum signals of the third plurality ofspread-spectrum signals. Also, the time delay would be different foreach chipping-sequence signal used to despread spread-spectrum signalsof the second plurality of spread-spectrum signals, and different fromeach time delay of each chipping-sequence signal used to despreadspread-spectrum signals of the first plurality of spread-spectrumsignals.

[0079] The method may be extended to a fourth, fifth or plurality ofgroups of spread-spectrum signals.

Variable Bandwidth Filter

[0080] The present invention also includes a variable-bandwidthspread-spectrum device for use with a spread-spectrum transmitter. Thevariable-bandwidth spread-spectrum device generates a spread-spectrumsignal having a spread bandwidth. The term “spread bandwidth”, as usedherein, denotes the bandwidth of the transmitted spread-spectrum signal.The variable-bandwidth spread-spectrum device uses a chipping-sequencesignal having a chipping rate which is less than the spread bandwidth.The term “chipping rate”, as used herein, denotes the bandwidth of thechipping-sequence signal.

[0081] The variable-bandwidth spread-spectrum device includes firstgenerating means, second generating means, spread-spectrum processingmeans, and filtering means. The spread-spectrum processing means iscoupled to the first generating means. The second generating means iscoupled between the spread-spectrum processing means and the filteringmeans.

[0082] The first generating means generates the chipping-sequence signalwith the chipping rate. The spread-spectrum processing means processes adata signal with the chipping-sequence signal to generate a spread-datasignal. The second generating means generates an impulse signal, inresponse to each chip of the spread-data signal. The filtering meansfilters the spectrum of each impulse signal with a bandpass equal to thespread bandwidth.

[0083] As illustratively shown in FIG. 16, the first generating meansmay be embodied as a chipping-sequence generator 161, the secondgenerating means may be embodied as an impulse generator 165, thespread-spectrum processing means may be embodied as an EXCLUSIVE-OR gateproduct device 164, or other device known to those skilled in the artfor mixing a data signal with a chipping-sequence signal, and thefiltering means may be embodied as a filter 166.

[0084] The product device 164 is coupled to the chipping-sequencegenerator 161. The impulse generator 165 is coupled between the productdevice 164 and the filter 166.

[0085] The chipping-sequence generator 161 generates thechipping-sequence signal with the chipping rate. The product device 164processes the data signal with the chipping-sequence signal, therebygenerating a spread-data signal as shown in FIG. 17. The impulsegenerator 165 generates an impulse signal, as shown in FIG. 18, inresponse to each chip in the spread-data signal shown in FIG. 17. Eachimpulse signal of FIG. 18 has an impulse bandwidth. The term “impulsebandwidth”, as used herein, denotes the bandwidth of the impulse signal.While theoretically an impulse signal has infinite bandwidth,practically, the impulse signal has a bandwidth which is greater thanthe spread bandwidth.

[0086] The filter 166 has a bandwidth adjusted to the spread bandwidth.Thus, the filter 166 filters a spectrum of each impulse signal of thespread-data signal with the spread bandwidth. The filter 166 does thisfor each impulse signal.

[0087] The filter 166 preferably includes a variable-bandwidth filter.The variable-bandwidth filter may be used for varying or adjusting thespread bandwidth of the spectrum for each impulse signal. Accordingly, aspread-spectrum signal may be designed having the bandwidth of choice,based on the bandwidth of the variable-bandwidth filter. The bandwidthmay be variable, or adjustable, as would be required for particularsystem requirements. As used in this patent, a variable bandwidth is onethat is able to vary according to time conditions, background signals orinterference, or other requirements in a particular system. Anadjustable bandwidth would be similar to a variable bandwidth, but isused to refer to a bandwidth which may be adjusted to remain at a chosensetting.

[0088] The first generating means, as shown in FIG. 19, may include afrequency-domain-chipping-sequence generator 161 and aninverse-Fourier-transform device 162. Thefrequency-domain-chipping-sequence generator 161 may be used to generatea frequency-domain representation of a chipping-sequence signal. Theinverse-Fourier-transform device 162 transforms the frequency-domainrepresentation of the chipping-sequence signal to the chipping-sequencesignal.

[0089] The first generating means may further include a memory 163 forstoring the chipping-sequence signal.

[0090] The present invention also includes a variable-bandwidthspread-spectrum method for use with a transmitter. The method includesthe steps of generating the chipping-sequence signal with the chippingrate, and spread-spectrum processing a data signal with thechipping-sequence signal to generate a spread-data signal. Each chip inthe spread-spectrum signal is used to generate an impulse signal. Eachimpulse signal is filtered with the spread bandwidth to generate thedesired bandwidth signal.

[0091] Thus, the variable-bandwidth-spread-spectrum device uses a lowerchip rate, but provides a wider bandwidth signal. The power spectraldensity at the output of the filter 166 of the filtered-spread-datasignal s(t) is proportional to the frequency response H(f) of thefilter.

PSD _(s(t)) −k|H(f)|²

[0092] Thus, the filter 166 controls the shape of the spectrum of thefiltered-spread-data signal.

[0093] The processing gain (PG) is bandwidth W of thefiltered-spread-data signal divided by chip rate Rb of thefiltered-spread-data signal.

PG=W/R _(b)

[0094] The capacity N of the filtered-spread-data signal is$N \leq {\frac{PG}{E_{b}/N_{0}} + 1}$

[0095] The capacity does not depend on chip rate, but instead onbandwidth. One can achieve an upper bound on the capacity if the chiprate is greater than the bandwidth. But, if the chip rate were lower,then one can save power consumption, i.e., use a lower clock rate ofCMOS, which determines power consumption.

Adaptive Power Control System

[0096] The present invention assumes that a plurality of mobile stationsoperate in a cellular-communications network using spread-spectrummodulation. The cellular communications network has a plurality ofgeographical regions, with a multiplicity of cells within eachgeographical region. The size of the cells in a first geographicalregion may differ from the size of the cells in a second geographicalregion. In a first geographical region, such as an urban environment,the cellular architecture may have a large number of cells, each ofsmall area, which place the corresponding base station close to eachother. In a second geographical region, such as a rural environment, thecellular architecture may have a smaller number of cells, each of largerarea. Further, the size of the cells may vary even within a specifiedgeographic region.

[0097] A mobile station, while in the urban environment of the firstgeographical region, may be required to transmit at a lower power levelthan while in the rural environment of the second geographical region.This requirement might be due to a decreased range of the mobile stationfrom the base station. Within a particular cell, a mobile station nearthe base station of the cell may be required to transmit with a powerlevel less than that required when the mobile station is near an outerperimeter of the cell. This adjustment in power level is necessary toensure a constant power level is received at the base station from eachmobile station.

[0098] Adaptive power control works by measuring the received signal tonoise ratio (SNR) for each user and causing the user transmitted powerto vary in a manner to cause all users' SNR's to be equal to a commonvalue which will be adequate for reliable communication if the totalnumber of users and interference is less than system capacity. Whilethis assumes that all users are obtaining the same service, e.g., 32 kbsvoice data, it is a feature of the system described that differentservice options are supported for requesting users. This is done byadjusting the setpoint for each user independently.

[0099] There are two issues that arise when addressing the baseoperation of an adaptive power control system. The first is the commonvalue obtained for SNR versus the load and its cost to the transmittersin terms of transmitted power, and the second is the stability of thesystem. Stability means that a perturbation of the system from itsquiescent state causes a reaction of the system to restore the quiescentcondition. It is highly desirable that there exist only one quiescentpoint because otherwise “chatter” or oscillation may occur. Stabilitymust be addressed with any control system but, in the present case, thesituation is somewhat complicated by the fact that the users affect oneanother, and thereby cause the control variables, the transmitted powerand resulting SNR's, to be dynamically coupled. The coupling is apparentwhen one realizes that all signals are processed by a common AGCfunction which does not discriminate individual user signals from eachother or from other sources.

[0100] The power control scheme of the present invention is a closedloop scheme. The system measures the correlator output power for eachuser and compares the measured value with a target value or setpoint.This measured power includes both the desired signal component andunwanted power or noise.

[0101] The AGC maintains the total power into each correlator at apreset level. This level does not vary as a function of APC action; thatis, this role of the AGC is independent of APC. Furthermore, an increasein received power from any user or subset of users will be “attacked” bythe AGC. This is possible because the AGC time constant is smaller thanthe APC time constant, i.e., the AGC is faster than the APC. Since thetotal power available out of the AGC is fixed, an increase in theportion due to one user comes at the expense of all other users. Whilethis may work against the apparent stability of the system, the AGCsensor, which measures the AGC control signal and thereby measures thetotal received power, causes the system to seek a quiescent statecorresponding to the minimum received power per user. It is desired thatthe transmitted power be minimized because this will minimize intercellinterference and conserve battery power. Excess transmitter power willbe dissipated within the AGC as long as all users transmit excessivepower.

[0102] The implementation shown in the figures is to be consideredrepresentative. In particular, the method of controlling the remotetransmitter power via attenuators and variable gain amplifiers isperhaps redundant. Either or both of these means may be employed,depending upon the (dynamic) range of control required. Also, controlmay be caused at either IF or RF frequencies.

[0103] For discussion purposes, a mobile station within a particularcell transmits a first spread-spectrum signal, and the base stationtransmits a second spread-spectrum signal. In the exemplary arrangementshown in FIG. 20, a block diagram of a base station as part of a systemfor adaptive-power control of a spread-spectrum transmitter is provided.

[0104]FIG. 20 illustrates the base station adaptive power control systemwith automatic gain control (AGC) means, power means, comparator means,transmitter means, and an antenna. The AGC means is shown as anautomatic-gain-control (AGC) amplifier 228, correlator means is shown asdespreader 231, and power means is shown as power measurement device233. The comparator means is shown as comparator 239, the transmittermeans is shown as power amplifier 237 coupled to the antenna 226. Alsoillustrated is a delta modulator 235 coupled between comparator 239 andpower amplifier 237.

[0105] The AGC amplifier 228 is coupled to the despreader 231. The powermeasurement device 233 is coupled to the despreader 231. The comparator239 is coupled to the output of the power measurement device 233 and tothe AGC amplifier 228. The multiplexer 234 is coupled between thecomparator 239 and the power amplifier 237. The delta modulator 235 iscoupled between the power amplifier 237 and the multiplexer 234. Thepower amplifier 237 is coupled to the antenna 226.

[0106] A threshold level is used by the comparator 239 as a comparisonfor the received-power level measured by the power measurement device233.

[0107] For each received signal, the AGC amplifier 228 generates anAGC-output signal and an AGC-control signal. The AGC-output signal isdespread to obtain the signal of a first user using despreader 231. Thedespread-AGC-output signal from the despreader 231 is combined with theAGC-control signal from the AGC amplifier 228, by the combiner 241. TheAGC-control signal from the AGC amplifier 228 may be offset by offsetlevel SI using combiner 242, and weighted by weighting device 243. Theweighting device 243 may be an amplifier or attenuator.

[0108] The received-power level from power device 233 may be offset byoffset level S2 using combiner 244, and weighted by weighting device245. The weighting device 245 may be an amplifier or attenuator. Thecombiner 241 combines the AGC-control signal with the received-levelsignal, for generating adjusted-received-power level. The comparator 239generates a comparison signal by comparing the adjusted-received-powerlevel to the threshold level. The comparison signal may be an analog ordigital data signal. The comparison signal indicates whether the mobilestation is to increase or decrease its power level. If theadjusted-received-power level exceeds the threshold, for example, thenthe comparison signal sends a message to the mobile station to decreaseits transmitter power. If the adjusted-received-power level were belowthe threshold, then the comparison signal sends a message to the mobilestation to increase its transmitter power. The comparison signal isconverted to a power-command signal by the delta modulator 235.

[0109] The power-command signal may be transmitted with or separate fromthe second spread-spectrum signal. For example, a spread-spectrum signalusing a first chip sequence may be considered a first spread-spectrumchannel, and a spread-spectrum signal using a second chip sequence maybe considered a second spread-spectrum channel. The power-command signalmay be transmitted in the same spread-spectrum channel, i.e., the firstspread-spectrum channel, as the second spread-spectrum signal, in whichcase the power-command signal is transmitted at a different timeinterval from when the second spread-spectrum signal is transmitted.This format allows the mobile station to acquire synchronization withthe first sequence, using the second spread-spectrum signal. Thepower-command signal may also be transmitted in a second spread-spectrumchannel which is different from the second spread-spectrum signal. Inthis case, the second spread-spectrum signal having the power-commandsignal would be acquired by the second chipping-sequence generator andsecond product device. The power-command signal may be time divisionmultiplexed or frequency division multiplexed with the secondspread-spectrum signal.

[0110] The base-correlator means is depicted in FIG. 20 as firstdespreader 231. The system, by way of this example, may have thebase-correlator means embodied as a product device, a chip-sequencegenerator, and a bandpass filter. Alternatively, the base-correlatormeans may be realized as a matched filter such as asurface-acoustic-wave device, or as a digital matched filter embodied ina digital signal processor. In general, the base-correlator means usesor is matched to the chip sequence of the spread-spectrum signal beingreceived. Correlators and matched filters for despreading aspread-spectrum signal are well known in the art.

[0111] Typically, the AGC circuit 228 is coupled to a low noiseamplifier 227, through an isolator 225 to the antenna 226. In FIG. 20 aplurality of despreaders, despreader 229 through despreader 231, areshown for despreading a plurality of spread-spectrum channels which maybe received from a plurality of mobile stations. Similarly, the outputof each despreader 229 through despreader 231 is coupled to a pluralityof demodulators, illustrated as demodulator 230 through demodulator 232,respectively, for demodulating data from the despread AGC-output signal.Accordingly, a plurality of data outputs are available at the basestation.

[0112] For a particular spread-spectrum channel, the first despreader231 is shown coupled to power device 233 and multiplexer 234. The powerdevice 233 typically is a power-measurement circuit which processes thedespread AGC-output signal as a received-power level. The power device233 might include an analog-to-digital converter circuit for outputtinga digital received-power level. The comparator means, embodied ascomparator circuit 239, compares the processed received-power level to athreshold. The multiplexer 234 is coupled to the output of the powerdevice 233 through the comparator circuit 239. The multiplexer 234 mayinsert appropriate framing bits, as required.

[0113] The transmitter means may be embodied as a quadrature phase shiftkeying (QPSK) modulator or a delta modulator 235 coupled to a poweramplifier 237. In FIG. 20, the input to the delta modulator 235typically would have the comparison signal from the comparator 239multiplexed with data from the k^(th) channel. The delta modulator 235converts the comparison signal to a power-command signal. A plurality ofspread spectrum channels would have their data and appropriatepower-command signals combined by combiner 236 and amplified by poweramplifier 237. The output of the power amplifier 237 is coupled throughthe isolator 125 to the antenna 226.

[0114] The power command signal is transmitted periodically. The periodT might be chosen to be 250 microseconds in order to ensure a low rootmean square error as well as a low peak error between the instantaneousreceived signal and the constant desired signal.

[0115] A mobile station is illustratively shown in FIG. 21. Themobile-despreading means is illustrated as despreader 334 andvariable-gain means is illustrated as a variable-gain device 341. Thevariable-gain device 341 is coupled between the transmitter 342 andthrough isolator 336 to antenna 335. The despreader 334 is coupled tothe isolator 336 and to demultiplexer 339. The output of the despreader334 is also coupled to a demodulator 340. The despreader 334 may beembodied as an appropriate correlator, or matched filter, fordespreading the k^(th) channel. Additional circuitry may be used, suchas radio frequency (RF) amplifiers and filters, or intermediatefrequency (IF) amplifiers and filters, as is well known in the art.

[0116] A received second spread-spectrum signal at antenna 335 passesthrough isolator 336 to despreader 334. The despreader 334 is matched tothe chip sequence of the desired spread-spectrum channel. The output ofthe despreader 334 passes through the demodulator 340 for demodulatingthe data from the desired spread-spectrum channel. Additionally, thedemultiplexer 339 demultiplexes the power-command signal from thedespread signal outputted from despreader 334. The power-command signaldrives the variable-gain device 341.

[0117] A decision device 345 and accumulator 346 may be coupled betweenthe demultiplexer 339 and the variable gain device 341. Astep-size-algorithm device 344 is coupled to the output of the decisiondevice 345 and to the accumulator 346.

[0118] The step-size-algorithm device 344 stores an algorithm foradjusting the power level of the variable gain device 341. A nonlinearalgorithm that might be used is shown in FIG. 22. FIG. 23 compares anonlinear algorithm with a linear step size algorithm.

[0119] The power-command signal from the demultiplexer 339 causes thedecision device 345 to increase or decrease the power level of thevariable gain device 341, based on the threshold of the step sizealgorithm shown in FIG. 23. The accumulator tracks previous power levelsas a means for assessing the necessary adjustments in the step size ofthe power level pursuant to the algorithm as shown in FIG. 23.

[0120] The variable-gain device 341 may be embodied as a variable-gainamplifier, a variable-gain attenuator, or any device which performs thesame function as the variable-gain device 341 as described herein. Thevariable-gain device 341 increases or decreases the power level of theremote station transmitter, based on the power-command signal.

[0121] As illustratively shown in FIG. 20, a block diagram of a powermeasurement circuit includes interference rejection for use with thebase station. As shown in FIG. 20, the AGC amplifier 228 is connected tothe despreader 231, and the output of the despreader 231 is connected tothe power measurement circuit 233. Additionally, the AGC amplifier 228is connected to the combiner 236 through the comparator 239.

[0122] A received signal includes a first spread-spectrum signal withpower P_(C) and the other input signals which are considered to beinterfering signals with power P_(J) at the input to the AGC amplifier228 of FIG. 20. The interfering signal may come from one or morenondesirable signals, noise, multipath signals, and any other sourcewhich would serve as an interfering signal to the first spread-spectrumsignal. The received signal is normalized by the AGC amplifier 228.Thus, by way of example, the AGC amplifier 228 can have the poweroutput, P_(C)+P_(J)=1. The normalized received signal is despread by thedespreader 231 to receive a particular mobile user's signal. Thechipping-sequence generator of despreader 231 generates a chip-sequencesignal using the same chip sequence as used by the first spread-spectrumsignal. Alternatively, the matched filter, if used, of despreader 231may have an impulse response matched to the same chip sequence as usedby the first spread-spectrum signal. The output of the despreader 231 isthe normalized power or the first spread-spectrum signal plus thenormalized power or the interfering signal divided by the processinggain, PG, of the spread-spectrum system. The power measurement circuit233 generates a received-power level of the first spread-spectrumsignal. The comparator 239 processes the despread-received signal withthe AGC-control signal and outputs the power-control signal of the firstspread-spectrum signal. The power level of the interfering signal isreduced by the processing gain, PG.

[0123] The comparator 239 processes the AGC-control signal with thedespread, normalized received signal by multiplying the two signalstogether, or by logarithmically processing the AGC-control signal withthe despread-received signal. In the latter case, the logarithm is takenof the power of the received signal, P_(C)+P_(J), and the logarithm istaken of the despread, normalized received signal. The two logarithmsare added together to produce the received-power level.

[0124] For the present invention to work effectively, the despreadsignal must be kept nearly constant, independent of variations in theother signals or of obstructions. A preferred implementation toaccomplish this end is shown in the circuitry of FIG. 20. FIG. 20depicts a means for determining at the base station the power of thefirst spread-spectrum signal when the received signal includes multiplesignals and noise. If the circuitry of FIG. 20 were not used, then it ispossible that the interfering signal, which may include noise, multipathsignals, and other undesirable signals, may raise the power levelmeasured at the input to the receiver of the base station, therebysuppressing the first spread spectrum signal. The undesirable powerlevel measured may cause the remote station to transmit more power thanrequired, increasing the amount of power received at the base station.

[0125] As noted earlier, the APC system is a closed loop system. The APCloop operates by generating commands to increase or decrease thetransmitter power at the update rate. This is actually quantizationprocess that is done to limit the amount of information that must be fedback to the remote transmitter. The amount of increase or decrease maybe fixed in advance or it may adapt in response to the characteristicsof the channel as measured locally in the remote terminal, the terminalbeing controlled. In particular, the remote terminal may examine thesequence of commands received by it. A long sequence of increasecommands, for example, implies that the step size may be increased. Atypical scheme increases the step size by a fixed amount or a fixedpercentage whenever two successive bits are the same. For example, thestep size may be increased by 50% if two bits in a row are the same anddecreased by 50% if they differ. This is a fairly gross change in thestep size, and is intended to be adaptive to local, or immediate intime, variations in the required transmitted power. This process resultsin a large variation of the step size with time.

[0126] An adaptive step size algorithm may also be considered in adifferent context. Specifically, the step size may be considered to benearly constant or not responding to localized variations in demandedtransmitted power, but the value may be automatically adjusted based onthe global characteristics of the channel induced control action. Thus,in a nearly static environment one should use a small constant step sizewhile in a mobile environment the step size should be larger.

[0127] Adjustment of the power level of the remote station transmittermay be effected either linearly or nonlinearly. The following algorithmwill cause the step size to settle at a nearly optimum constant value.The receiver examines successive APC bits and increases the step size bythe factor (1+x) if they agree and decreases the step size by the factor(1+x) if they disagree. Here the parameter x is small (x=0.01, orexample). While this procedure will not allow local adaptation (becausex is small), it will result in an adaptation to global conditions.Specifically, if the transmitted APC bit stream exhibits a tendencytoward successive bits in agreement (i.e., runs of 1's or 0'sare-evident) it implies that the system is not following the changes inchannel conditions (i.e., the system is slow rate limited) and the stepsize should be increased. On the other hand, if successive bits tend tobe opposite, the system is “hunting” for a value between two values thatare excessively far apart. The statistics one expects to observe addsoptimal are intermediate to these extremes. That is, the ADC bit streamshould appear equally likely to contain the patterns (0,0), (0,1),(1,0), and (1,1) in any pair of successive bits. The above algorithmdrives the system behavior toward this.

[0128] The above algorithm (global adaptation) works particularly wellwhen the system employs a high update rate relative to the dynamics ofthe channel.

[0129] As illustrated in FIG. 23, to increase the power level usinglinear adjustment, for example, the transmitter power is increased inregular increments of one volt, or other unit as instructed by the basestation, until the power level received at the base station issufficiently strong. Linear adjustment may be time consuming if thepower adjustment necessary were substantial.

[0130] As shown in FIG. 22, to increase the power using nonlinearadjustment, the transmitter voltage may be increased, by way of example,geometrically until the transmitted power is in excess of the desiredlevel. Transmitter power may be then reduced geometrically untiltransmitted power is below the desired level. A preferred approach is toincrease the step size voltage by a factor of 1.5 and to decrease thestep size by a factor of 0.5. Other nonlinear algorithms may be used. Asshown in FIG. 23, this process is repeated, with diminishing margins oferror in both excess and insufficiency of desired power, until thedesired signal level has been obtained. Nonlinear adjustment provides asignificantly faster rise and fall time than does linear adjustment, andmay be preferable if power must be adjusted significantly.

[0131] The system determines the error state (APC bit) every T sections,1/T being the update rate of the control. The update rate may vary from100 Hz, which is low, to 100 kHz, which is quite high. The opportunityto measure the error state of the system arises with each reception of anew symbol. Thus, the update rate may be equal to the symbol rate. Ifsuch an update rate is not supported, it is beneficial to make use ofthe available error measurements by combining them (or averaging them)between updates. This minimizes the chance of causing a power adjustmentin the wrong direction which can occur because of noise in the errorsignals themselves.

[0132] The choice of update rate depends on factors other than APCoperation, namely, the amount of capacity and method of allocatingcapacity to the transport of the APC bits over the channel. In general,a faster update will produce superior performance, even if the increasedupdate rate is obtained by permitting the APC bits to be received inerror occasionally. Elaborating, a 1 kHz update rate with no channelinduced errors will perform less effectively than a 100 kHz update rateat a 25% rate of errors. This is because of the self correcting behaviorof the control loop. A faster update rate eliminates the latency ofcontrol which is a key performance limiting phenomenon.

[0133] A spread spectrum base station receives all incoming signalssimultaneously. Thus, if a signal were received at a higher power levelthan the others, then that signal's receiver has a highersignal-to-noise ratio and therefore a lower bit error rate. The basestation ensures that each mobile station transmits at the correct powerlevel by telling the remote, every 500 microseconds, whether to increaseor to decrease the mobile station's power.

[0134]FIG. 24 shows a typical fading signal which is received at thebase station along with ten other independently fading signals andthermal noise having the same power as one of the signals. Note that thefade duration is about 5 milliseconds which corresponds to vehicularspeed exceeding 60 miles per hour. FIGS. 25-26 illustrate the resultsobtained when using a particular adaptive power control algorithm. Inthis case, whenever the received signal changes power, the base stationinforms the remote and the remote varies its power by ±1 dB. FIG. 25shows the adaptive power control signal at the remote station. FIG. 26shows the received power at the base station. Note that the adaptivepower control track the deep fades and as a result 9 dB fades resulted.This reduced power level resulted in a bit error rate of 1.4×10⁻².

[0135] For the same fade of FIG. 24, assume a different adaptive powercontrol algorithm is employed as shown in FIGS. 27-28. In this case thecontrol voltage results in the remote unit changing its power by afactor of 1.5 in the same direction, or by a factor of 0.5 in theopposite direction. In this particular implementation the minimum stepsize was 0.25 dB and the maximum step size was 4 dB. Note that the erroris usually limited to .+−.2 dB with occasional decreases in power by 5dB to 6 dB resulting in a BER≠8×10⁻⁴ , a significant improvementcompared to the previous algorithm. The use of interleaving and forwarderror correcting codes usually can correct any errors resulting from therarely observed power dips.

[0136] In operation, a mobile station in a cell may transmit the firstspread-spectrum signal on a continuous basis or on a repetitive periodicbasis. The base station within the cell receives the firstspread-spectrum signal. The received first spread-spectrum signal isacquired and despread with the chip-sequence signal from chip-sequencegenerator and product device. The despread first spread-spectrum signalis filtered through bandpass filter. The base station detects thedespread first spread-spectrum signal using envelope detector, andmeasures or determines the received-power level of the firstspread-spectrum signal. The base station generates the power-commandsignal from the received-power level.

[0137] The present invention also includes a method for automatic-powercontrol of a spread-spectrum transmitter for a mobile station operatingin a cellular-communications network using spread-spectrum modulation,with the mobile station transmitting a first spread-spectrum signal. Inuse, the method includes the step of receiving a received signal,generating an AGC-output signal, despreading the AGC-output signal,processing the despread AGC-output signal to generate a received-powerlevel, generating a power-command signal, transmitting the power-commandsignal as a second spread-spectrum signal, despreading the power-commandsignal from the second spread-spectrum signal as a power-adjust signal,and adjusting a power level of the first spread-spectrum signal.

[0138] The received signal includes the first spread-spectrum signal andan interfering signal and is received at the base station. TheAGC-output signal is generated at the base station and despread as adespread AGC-output signal. The despread AGC-output signal is processedat the base station to generate a received-power level.

[0139] The received-power level is compared to a threshold, with thecomparison used to generate a power-command signal. If thereceived-power level were greater than the threshold, the power-commandsignal would command the mobile station to reduce transmitter power. Ifthe received-power level were less than the threshold, the power-commandsignal would command the mobile station to increase transmitter power.

[0140] The power-command signal is transmitted from the base station tothe mobile station as a second spread-spectrum signal. Responsive toreceiving the second spread-spectrum signal, the mobile stationdespreads the power-command signal as a power-adjust signal. Dependingon whether the power-command signal commanded the mobile station toincrease or decrease transmitter power, the mobile station, responsiveto the power adjust signal, increases or decreases the transmitter-powerlevel of the first spread-spectrum signal, respectively.

[0141] The method may additionally include generating from a receivedsignal an AGC-output signal, and despreading the AGC-output signal. Thereceived signal includes the first spread-spectrum signal and aninterfering signal. The received signal is processed with the despreadAGC-output signal to generate a received-power level. The method thengenerates a comparison signal by comparing the received-power level tothe threshold level. While transmitting a second spread-spectrum signal,the method adjusts a transmitter-power level of the firstspread-spectrum signal from the transmitter using the power-adjustsignal.

[0142] It will be apparent to those skilled in the art that variousmodifications can be made to the spread-spectrum system and method ofthe instant invention without departing from the scope or spirit of theinvention, and it is intended that the present invention covermodifications and variations of the spread-spectrum system and methodprovided they come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A system for adaptive-power control of a spread-spectrum transmitter of a mobile station operating in a cellular-communications network using spread-spectrum modulation, comprising: a mobile station having a mobile transmitter coupled to a mobile antenna for transmitting a first spread-spectrum signal using radio waves; a base station having an automatic-gain-control (AGC) circuit, responsive to a received signal, for generating an AGC-output signal, with the received signal including the first spread-spectrum signal and an interfering signal; a base correlator coupled to said AGC circuit for despreading the AGC-output signal; a power-measurement circuit coupled to said base-correlator, for processing the despread AGC-output signal as a received-power level; a comparator coupled to said power-measurement circuit for generating a comparison signal using a threshold test and the received-power level; a power-command circuit coupled to said comparator and responsive to the comparison signal for generating a power-command signal; an antenna; a transmitter coupled to said antenna and to said power-command circuit for transmitting the power-command signal as a second spread-spectrum signal; and said mobile station further including a mobile correlator coupled to said mobile antenna for despreading the power-command signal from the second spread-spectrum signal as a power-adjust signal; a decision device coupled to said mobile correlator for responding to the power-adjust signal; an accumulator coupled to said decision device; a step-size algorithm device coupled to said decision device and to said accumulator for storing an algorithm for adjusting a power level; and a first variable-gain device coupled to said accumulator, responsive to the power-adjust signal, for adjusting a transmitter-power level of the first spread-spectrum signal transmitted from said mobile-transmitter.
 2. The system of claim 1 with said power-measurement circuit coupled to said AGC circuit and to said base-correlator, responsive to processing the received signal with the despread AGC-output signal, for generating the received-power level.
 3. The system of claim 1 with said first variable-gain device adjusting the transmitter-power level using nonlinear steps in power level.
 4. The system of claim 1 wherein a power control scheme of the system is based on a signal to noise ratio of the first spread-spectrum signal.
 5. A system for adaptive-power control of a spread-spectrum transmitter of a mobile station operating in a cellular-communications network using spread-spectrum modulation, with the mobile station transmitting a first spread-spectrum signal, said system comprising: a base station having, automatic gain control (AGC) means, responsive to a received signal, with the received signal including the first spread-spectrum signal and an interfering signal, for generating an AGC-output signal and an AGC-control signal; base-correlator means for despreading the AGC-output signal as a despread AGC-output signal; power means for processing the despread AGC-output signal as a received-power level; comparator means for generating a comparison signal using a threshold test and the received-power level; modulator means for converting the comparison signal to a power command signal; and transmitter means for transmitting the power-command signal as a second spread-spectrum signal; a mobile station having, despreader means for despreading the power-command signal from the second spread-spectrum signal as a power-adjust signal; decision means, responsive to the power-adjust signal, for increasing or decreasing a power level; and variable-gain means, responsive to said decision means, for adjusting a transmitter-power level of the first spread-spectrum signal.
 6. The system of claim 5 with said power means responsive to processing the received signal with the despread AGC-output signal, for generating the received-power level.
 7. The system of claim 5 with said variable-gain means adjusting the transmitter-power level using nonlinear steps in power level.
 8. The system of claim 5 with said variable-gain means adjusting the transmitter-power level using nonlinear steps in power level.
 9. The system of claim 5 wherein said AGC means includes an automatic-gain-control circuit.
 10. The system of claim 5 wherein said variable-gain means includes a variable-gain attenuator for adjusting the transmitter-power level.
 11. The system of claim 5 wherein said variable-gain means includes a variable-gain amplifier for adjusting the transmitter-power level.
 12. The system of claim 5 wherein a power control scheme of the system is based on a signal to noise ratio of the first spread-spectrum signal.
 13. A method for adaptive-power control, from a base station, of a spread-spectrum transmitter of a mobile station operating in a cellular-communications network using spread-spectrum modulation, with the mobile station transmitting a first spread-spectrum signal, said method comprising the steps of: generating at said base station, from a received signal, with the received signal including the first spread-spectrum signal and an interfering signal, an AGC-output signal; despreading, at said base station, the AGC-output signal as a despread AGC-output signal; processing, at said base station, the despread AGC-output siganl to generate a received-power level; generating, at said base station, a comparison signal using a threshold test and the received power level and generating a power command signal responsive to said comparison signal; transmitting, from said base station, the power-command signal as a second spread-spectrum signal; despreading, at said mobile station, the power-command signal from the second spread-spectrum signal as a power-adjust signal; and adjusting, at said base station, responsive to the power-adjust signal, a transmitter-power level of the first spread-spectrum signal.
 14. The method of claim 13 wherein the step of adjusting includes the step of adjusting the transmitter-power level using nonlinear steps in power level.
 15. The method of claim 13 wherein a power control scheme of the method is based on a signal to noise ratio of the first spread-spectrum signal. 